When a voice call is made using a conventional analog subscriber set, only a small portion of the frequency spectrum available on the typical subscriber line is utilized. During the development of distributed data processing systems, an effort was made to utilize the existing subscriber line network to facilitate communication of the digital data used by such systems. Various types of modulator/demodulator (modem) devices have been developed to take advantage of the latent capability of the subscriber line network to support digital data communication at data rates significantly greater than that required for analog voice communication. For example, synchronous modems are commercially available which utilize the differential phase shift keyed (DPSK) modulation/demodulation technique to provide data communication at rates up to 9600 baud. However, even in the digital private automatic branch exchanges (PABX's) which support intra-exchange communication of voice information in the form of digital PCM voice data words, the response characteristics of the several analog components of the typical analog line card used therein generally prevent reliable communication at higher rates.
Several techniques have been proposed for providing higher speed digital data communication between subscribers through such PABX's. However, such proposals typically require the installation of additional pairs of subscriber lines to subscribers requiring the service and/or redesign/modification of the particular PABX to provide the capability to process the pure digital data words in a manner different from the digital PCM voice data words.